Relatively small vacuum diffusion pumps designed to evacuate a chamber to a vacuum of 10.sup.-7 torr, or to a greater vacuum, have typically been relatively high priced items, particularly if they have been designed to have good speed characteristics and high fore-pressure tolerances, on the order of 0.5 torr. A typical prior art diffusion pump is illustrated in FIG. 1 and includes a cylindrical body assembly 11, having a longitudinal axis 12. Body assembly 11 includes a cylindrical shell 13, coaxial with axis 12, fabricated from roll-up or standard tubing cut to size. At the bottom of shell 13, a pool 14 of vaporizable diffusion pump fuild or oil is provided. The peripheral wall of pool 14 is formed by the bottom of shell 13, while the floor of the pool is defined by an annular, machined metal plate 15 that is bonded, usually by welding, to the bottom edge of shell 13. In one configuration, the liquid in pool 14 is heated to vaporization by an electric resistance heater 16 that is inserted in tubular, sheet metal thimble 17. A protective cup 18 extends downwardly from plate 15 and includes a connector 19 to which leads for heater 16 are connected to an external power source. In certain configurations, axially extending heater 16 and thimble 17 are replaced with a heater assembly mounted below machined metal plate 15.
The diffusion pump is sealingly connected through an opening to baffle assembly 71 and vacuum chamber 21. To this end, the top of shell 13 is welded to machined metal flange plate 22 that is concentric with axis 12 and includes an annular groove in which sealing O-ring 23 is placed. Flange plate 22 has a plurality of bores which mate with corresponding bores in a flange of baffle assembly 71; bolts 24 are inserted in the matching bores to sealingly connect the diffusion pump to baffle assembly 71 which, in turn, is connected to chamber 21 through a flange and seal arrangement similar to that described for the connection between the pump and baffle assembly.
Within body assembly 11 a jet assembly (not shown in FIG. 1) is provided; the jet assembly is made by spinning or from machined castings and includes at least one, and usually several, annular diffusion pump nozzles for directing vapor evaporated from pool 14 downwardly against shell 13. The diffusion pump fluid contacting shell 13 is condensed since the shell is cooled by a cooling coil 25 having a uniform pitch along the portion of shell 13 just below flange 22 to a region just above radially extending conduit 26 that forms a portion of fore line 27 that is connected to a fore line pump (not shown). Conduit 26 is positioned opposite from a horizontally directed ejector stage nozzle (not shown in FIG. 1).
To prevent migration of diffusion pump vapors from the diffusion pump into vacuum chamber 21, a baffle assembly 71 is usually inserted just above flange 22 and below flange 73 that connects the baffle assembly to chamber 21. Assembly 71 includes blocking plate 74 that is mounted on cooling coil 75 in a horizontal plane at right angles to axis 12 to prevent axial flow of vapor between the pump and chamber.
From a review of FIG. 1, it becomes apparent that the prior art device is relatively expensive to build and operate. In particular, numerous operations are required to assemble body assembly 11, in that it is necessary to machine the plates forming pool bottom 15 and flange 22 and to then weld these plates completely around their peripheries to form seals with the top and bottom edges of shell 13.
In addition, the typical prior art design of cooling coils 25, so that the coil has a uniform pitch, results in a substantial cost factor, insofar as materials of the cooling coil and the cost of installation are concerned. The typical prior art design has also tended to cause more cooling than is necessary so that the diffusion pump fluid has a tendency to be excessively cool when it reaches pool 14. Excessive cooling of the diffusion pump fluid, when it reaches pool 14, increases the cost required to operate heater 16, in addition to the cost of energy required to operate coil 25.
The typical, previously utilized baffle also adds to the expense of the unit by increasing material and labor costs. In addition, fabricating the jet assembly from spinning or machine castings has a tendency to increase costs, particularly if high production is expected.